Biology Reference
In-Depth Information
eight different samples (soil and water) obtained from Lake Somerville (N30°30′09″
and E96°64′28″), Brazos River (N30°55′84″ and E96°42′24″; N30°62′64″ and
E96°55′13″) and Lake Finheather (N30°64′93″ and E96°37′54″) around College Sta-
tion, Texas.
Isolation and Pre-screening of Environmental Microbes
We performed a pre-screening for electrochemically active microbes. Each diluted
sample was plated on nutrient agar and incubated under anaerobic conditions. The
resulting 50-100 microbial colonies per plate were then used for plating on nutrient
agar containing 100 μM Reaction Black 5, an azo dye that resulted in dark blue color
of the media. After 3 days of incubation, a total of 26 colonies formed discoloration
halos out of about 1,500 colonies plated for each of the eight environmental samples.
The discoloration of the dye indicated reduction capability of the microbes. A total of
13 isolates were selected for MFC array screening. Un-inoculated medium was used
as the negative control.
The 16s rDNA Amplification and Phylogenetic Analyses for Environmental
Isolates
Colony PCRs were performed using different environmental isolates as the templates.
The PCR products were then purified and sequenced with primers 11F and 1492R.
The 16S rDNA sequences were BLAST searched against the GenBank database and
the top hit for each isolate were used for alignment and phylogenetic tree generation.
Sequences of the 16S rDNA of 15 members of genus
Shewanella
similar to 7Ca were
aligned and phylogenetic tree was constructed among selected
Shewanella
. A matrix
of pairwise genetic distances by the maximum-parsimony algorithm and the neighbor-
joining method was used to generate phylogenetic trees.
The MFC Array Characterization and Data Acquisition
Two characterization methods were used to evaluate electricity generation from each
of the 24 MFC wells. First, 24 1 MΩ fixed load resistors were connected to each of the
MFC wells and voltage across these resistors were recorded. Load resistance of 1 MΩ
was selected for MFC characterization because power output of
S. oneidensis
MR-1 at
this resistance was close to maximum and the fabricated MFC array showed a steady
state current output much faster than using resistors with lower resistances. A switch
box module having an integrated digital multimeter (PXI-2575, PXI-4065, National
Instruments, Austin, TX) and controlled through LabView™ (National Instruments,
Austin, TX) was used to continuously measure voltages across the 24 load resistors
that were connected individually to the 24 MFC array. The measured voltages were
converted to current densities (mA/m
2
, electrode area: 0.385 cm
2
) using Ohm's law,
and power densities were calculated using P = VI/A (V: voltage, I: current, A: elec-
trode area).
Full characterization of an MFC requires a current density versus power density
plot, which can be obtained when measuring voltages across varying resistors. In this
second characterization method, twenty-four 100 KΩ variable resistors (652-3296Y-
1-104LF, Mouser Electronics, Mansfi eld, TX) were connected in series with twenty-four
